The present invention generally relates to a phytoremediation process for removing shallow subsurface contaminants in soils. More particularly, the present invention provides a process which combines surface chemical reaction (i.e., fixation and degradation) of contaminants with the phytoextraction and subsequent accumulation of contaminants in the above-ground biomass of the plants. In particular, a combination of phytoextraction and surface chemical reaction is provided which allows passive remediation over numerous plant growth cycles to occur without the expense of above-ground biomass removal and disposal.
Phytoextraction is a recognized method of reducing soil contamination. Phytoextraction refers to a natural growth process in which soil minerals and ions are actively accumulated within plant tissues at concentrations greater than that accountable by simple concentration gradients. Accordingly, some form of active transport occurs where roots accumulate organic and inorganic compounds through normal metabolic pathways. Certain species (hyperaccumulators) of plants show high affinity for select minerals or ions. Such species have been used in soil remediation efforts in which phytoaccumulating plants are grown in contaminated soil.
One such method, disclosed in U.S. Pat. No. 5,720,130 to Bost, and which is incorporated herein by reference, teaches that selected plants may accumulate metals, metal salts, nonmetals, and organic waste which migrate from the roots and into the above-ground parts of the plant. Thereafter, the plants are harvested and the contaminated plant material removed.
It is also known to increase the rate of phytoaccumulation within select plants by the addition of soil amendments. One such method is taught in U.S. Pat. No. 5,711,784 to Chaney et al., which is incorporated herein by reference. In Chaney et al., the soil amendments are added to promote the uptake by the plants of selected contaminants. Thereafter, the above-ground biomass of the plants is harvested, removed, and disposed of as a hazardous material.
Currently, a need exists for a phytoremediation process which avoids the cost and expense of harvesting and disposing of above-ground biomass. Also is protective of or less disruptive to sensitive ecosystems, i.e., wetlands. In particular, there exists a need for a phytoextraction and accumulation process which allows repeated growth cycles of the phyotaccumulating plants to occur and in which the dead or harvested biomass may be left in place.
It is an object of the present invention to provide an improved phytoextraction/chemical fixation process for use in remediation of shallow soil contaminants.
Another object of the present invention is provide a phytoaccumulation process in which senescent biomass may be left in place without further migration of the contaminants.
It is yet another object of the present invention to provide a treatment process for contaminants which includes undergoing degradation following chemical fixation. In other words, organic compounds such as PCBs and TNT may be chemically fixed by the phytoaccumulation process and then subject to a specific [microbial] degradation by the introduction of suitable biological amendments.
These and other objects of the present invention are achieved by a process of providing a chemically and physically stabilized amendment layer along an upper soil surface. Following the amendment of the top soil layer, phytoextracting plants along with indigenous plants are introduced into the soil region within the contaminated site. As the plants grow, the plants extract and accumulate contaminants which are transported, in part, to the above-ground portions of the plant. The plants, particularly if annuals, may remain in situ without further handling and the natural senescence process may proceed undisturbed. Alternatively, and depending upon the accumulated contaminant, the plants may be harvested, i.e., cut, up-rooted, treated with herbicides, or subjected to a prescribed burn. In all cases, the plant material harvested remains at the original growth site.
The upper soil surface amendments provide a stabilizing layer for the contaminants in the above-ground biomass. Repeated growth cycles may occur naturally or through re-seeding or otherwise repopulating the growth site. The cycle(s) may be repeated as needed until the subsurface contaminant concentrations have been reduced to acceptable levels.
Thereafter, the amended soil layer and contaminated biomass may be removed for disposal. Alternatively, if the chemically fixed contaminants are sufficiently stable, the plant biomass, accumulated contaminants and the amended soil layer may be left in situ or capped with an additional layer of amended soil or barrier mats. Additionally, a biologically active agent such as specific bacteria may form part of the amendments so as to provide a biological degradation of any contaminants.
In general, any application which makes use of phytoextraction as a treatment protocol for shallow surface contaminants, may be used in conjunction with a stabilized upper surface soil layer which may be provided by soil amendments and/or prepared barrier mats designed to react and/or chemically fixate the contaminants upon contact. In some embodiments, the phytoextracting plants may be left undisturbed to undergo natural growth and senescence cycles. In other embodiments, the above-ground biomass may be removed at ground level but otherwise left in place. In yet another embodiment, the stabilized soil amendments provides a permanent treatment protocol for all contaminants brought to the upper surface level by the phytoaccumulating plants. In yet another embodiment, once the inorganic or organic contaminants have been brought to the upper surface, the amended soil layers and accumulated biomass from the phytoaccumulators may be removed for subsequent treatment or disposal.
Other objects, features and aspects, of the present invention are discussed in greater detail below.